Write protect control circuit for computer hard disc systems

ABSTRACT

A write protect system prevents inadvertent attempts to write data into a permanent memory storage device (such as a disc drive) containing valuable data that might be destroyed by such a write attempt. The write protect system includes a controller coupled to at least one memory storage device via information transfer busses and a control data bus. The controller sends signals for writing data into selected ones of memory devices. A circuit is coupled into the control data bus for monitoring signals transmitted thereover. Responsive to the monitoring circuit, access is barred to preclude writing into a specific one of the memory devices whereby data may not be accidentally written into the specific memory. The barring is cancelled if a specific action is taken whereby data may be deliberately written into a specific memory.

This invention relates to control circuits for computers andcomputer-like equipment and, more particularly, to write protection forsuch equipment having a plurality of means for storing data.

The basic problem which the invention addresses is one of preventingaccidental interference with previously stored data responsive to anincorrectly attempted storage of data in the wrong memory of a computer.For example, suppose that valuable data is already permanently stored ona first and hard disc. Also, suppose that someone wants to temporarilyenter new data on a second and floppy disc. Both discs are accessed bythe computer in a similar manner, but by using slightly different memorycodes. There always is a chance that human or equipment error mightcause the computer to be operated in a manner which alters or destroysthe valuable permanent data stored on the first or hard disc, instead ofentering the new data on the second or floppy disc.

Thus, there is a need to "write protect" the first disc, which meansthat a circuit is activated which absolutely bars access to the firstdisc, during the writing of data onto the second disc. If there is aneed to enter data in the first or protected disc, the write protectcircuit must be off. That is a deliberate act which will not beaccidentally undertaken.

More particularly, for many years, large main frame computers have usedsecondary memories in the form of tape drives and disc drives. Both ofthese types of devices often have two useful switches for giving writeprotect and online control. Since most computers have several tape ordisc drives, it is not unusual to be performing an operation which isreading from one device and writing into another device. To guardagainst accidents, the drive for the device from which data is beingread is "write protected" responsive to the throwing of a switch on thefront of a drive for the read device. A similar safeguard is a switchlabeled "online" which may be turned off to completely detach that readdevice from the computer.

When floppy disc technology was introduced for mini and micro computers,the write protect device took the form of a notch in the floppy discitself. The drive may optically sense whether or not the notch iscovered and, therefore, decide if the disc is "write protected." Again,this technique guards against accidents in the nature of over-writingvaluable data.

Accordingly, an object of the invention is to provide new and improvedmeans for giving write protection to a disc drive. Here, an object is toprovide simple and low cost equipment which may be added onto virtuallyany computer-like equipment. In this connection, an object is to enablethe inventive write protect circuit to be installed on almost anycomputer or similar device without requiring any modification thereof.

Another object of the invention is to provide a write protect device forpersonal and business computers which cannot be overriden throughaccident.

In keeping with an aspect of the invention, these and other objects areaccomplished by a simple logic circuit which observes when write signalsaddressed to a particular device appear on a data bus. If the writesignals appear at a time when the particular device is enabled to write,a write request or gate signal is passed so that the writing may beaccomplished. If the write signals appear on the data bus at a time whenthe particular device is not enabled, the write request or gate signalis inhibited and the drive may not write any data. Therefore, thevaluable data already stored on the protected device cannot bedisturbed.

A preferred embodiment of the invention is shown in the attacheddrawings, wherein:

FIG. 1 is a block diagram of a prior art system, including a computer orcomputer-like device, having two associated disc drives;

FIG. 2 is the same system with the inventive write protect deviceinstalled in a control bus;

FIG. 3 is a block diagram of the interconnections between the inventivedevice and two disc drives; and

FIG. 4 is a logic circuit diagram of the inventive device.

The system of FIG. 1 includes a controller 20, which may be any suitabledevice. By way of example, a pair of disc drives 22, 24 are shown asbeing connected to the controller 20 via commercially available databusses 26, 28 for the transmission of information signals. The discdrives 22, 24 are also connected to the controller 20 via a commoncontrol bus 30. All of the busses are two-way data transfer devices.

For convenience of this description, it is assumed that both of thedrives 22 and 24 are Winchester fixed disc systems, although theinvention may be applied to any other suitable equipment. The controller20 and drives 22, 24 are associated with a small personal or businesscomputer. The devices and busses use industry standards for makinginterface connections. If some other drive, or some other computer orinterface standard is used, the principles explained herein may beeasily modified to fit those devices or standards.

According to the invention (FIG. 2), the control data bus 30 isdisconnected from the controller 20 and connected to a monitoring meansin the form of write protect device 32. A new control data bus 34 isconnected between the controller 20 and the write protect monitoringdevice 32. All other equipment in FIG. 2 remains the same as those shownin FIG. 1. The write protect monitoring device 32 enables data which issent over information data bus 26 to be written in disc drive device 22,unless device 32 is first activated in a manner which disables suchwriting.

The pertinent ones of the specific interconnections provided by the twocontrol busses 30, 34 are shown in FIG. 3. While these connections areshown separately from the circuit 32, they are preferably internallywired so that the connections are made by simply connecting a data businto each of two opposite sides of a box, so to speak.

The central processor of a business or personal computer 40 is coupledto a computer bus 42 which may extend to any suitable equipment, whichmay be used for any suitable purpose. A host interface 44 is used toconnect a controller 20 to the computer bus 42. All of these items 40-44and 20 (and anything else connected to bus 42) are standard commercialitems.

The control data bus 34 includes a "device select 1" wire 35 whichselects disc drive device 22 and a "device select 2" wire 36 whichselects disc drive device 24. One, and only one, of these device selectwires is activated at any given time; therefore, only one of the discdrive devices may be activated at any given time. The fault would occurif the "enable device 1 protection" switch 48 is activated at a timewhen the intention is to enter data in device 22.

The "write fault" wire 37 is one which is activated by either of thedevices 22, 24 if, for any reason, it cannot accept the data being sentto it. The write protect circuit 32 may simulate this write faultsignal. Regardless of its source, an appearance of a fault signal on the"write fault" wire 37 informs the controller 20 that the data was notwritten as requested.

The "write gate" wire 38, 39 carries an enable signal which causes theselected disc drive to write. Thus, for example, if the select wire 35is activated, disc drive 22 is enabled and placed on standby. When asignal also appears on the "write gate" wire 38, 39, the disc drive 22records whatever information is then being sent over information databus 26 (FIG. 2).

The write protect device 32 is coupled to monitor the signals appearingon the device select wires 35, 36. The write gate signals on wire 38pass through write protect device 32 to wire 39. If there is a mistakeneffort to write into the wrong disc drive at a time when it isprotected, the signal on the write gate wire 38 is not passed throughwrite protect circuit 32 to wire 39, and a signal is applied at 47 tothe write fault wire 37.

A manual "enable device 1 protection" switch 48 is provided on thedevice 32. This switch is operated whenever there is a need to protector unprotect device 22. Another switch "enable device 2 protection"might have been provided if protection was required for device 24.

Three lamps (which may be LED's) 50-52 visually indicate when theprotected device is selected, when it is protected, and when an effortis being made to write into it. These lamps provide for humansupervision over the write operation.

The details of the write protect circuit 32 are shown in FIG. 4. If thecircuit is to be activated, the "enable device 1 protection" switch 48is activated. Responsive thereto, the inverter 56 is activated to lightLED 51, which indicates that the associated disc drive device isprotected. Also, responsive to the closure of switch 48, the lower inputof a two input NAND gate 58 is marked, without immediate effect.Resistor 60 provides a bias voltage and resistor 62 limits current.

When the disc controller 20 attempts to write into the protected discdrive device 22, a signal appears on the "device select 1" wire 35 andpasses through the inverter 66 to the upper input of NAND gate 58. Thereis a coincidence with the signal from switch 48 so that NAND gate 58conducts. The inventive device is now in the so-called "protecting mode"for device 1. Also, responsive to the output of inverter 66, a signal isfed throught inverter 68 to light LED 50, thus indicating that theprotected device 22 has been selected.

The output of NAND gate 58 passes through inverter 70 thereby removingthe inhibit from the latch circuit 72, allowing the transmission of awrite fault signal. The output of NAND gat 58 also applies a disablesignal to a guard means in the form of NAND gate 74. The circuit is nowready to intercept any write gate signal and generate a write fault.

As the undesired write operation continues, a signal appears on the"write gate" wire 38. Resistors 76, 78 form a voltage divider forsetting an input bias for inverter 80. The write gate signal on wire 38activates the inverter 80 to change its state.

The output of inverter 80 passes to the upper input of NAND gate 74.Since the output of NAND gate 58 is disabling the activation of NANDgate 74, the "drive write gate" signal is not asserted on wire 39 anddoes not enable writing on disc drive device 22.

The output of inverter 80 also triggers a pulse generating circuit 82 tolight LED 52 which indicates that a write request has occurred. Resistor84 is current limiting. Also, responsive to the output of inverter 80,the flip-flop 72 is activated and a fault signal is transmitted throughan inverter 88 to the "write fault" wire 37. From FIG. 3, a signal onthe fault wire informs the disc controller that the write operation inprogress has failed. At the end of the write sequence, the disccontroller device 20 de-asserts the "device 1 select" signal on wire 35which resets the flip-flop 72 by way of the path 35, 66, 58, 70 therebyterminating the "write fault" signal on wire 37.

Thus, when the switch 48 is closed, any information transmitted over thedata bus 26 (FIG. 2) is not written into the protected device 22.

If device 24 is selected (a non-protected device), there is no signal onwire 35, and inverter 66 and NAND gate 58 are not activated. Inverter 70does apply an inhibit signal to flip-flop 72 for disabling thegeneration of write fault conditions. If a write gate signal appears atinput 38, inverter 80 is activated, pulse generator 82 operates and LED52 lights. The output of inverter 80 is applied to the upper input ofNAND gate 74 coincidental with an enable on the lower input therebycausing NAND gate 74 to assert a "drive write gate" signal on wire 39 todevice 24. Thus, device 24 is write enabled and unaffected by the writeprotected state of device 22.

If the circuit 32 is not to be activated, switch 48 is not activated.When the switch 48 is not activated, the bias voltage caused by resistor60 disables AND gate 58. Independent of the presence of any selectsignals, the circuit is now said to be in "non-protecting mode." Thealways disabled NAND gate 58 (i.e. when switch 48 is open) enables NANDgate 74. Therefore, the presence of a "controller write gate" at wire 38always creates a "drive write gate" signal at wire 39. The inactiveoutput of NAND gate 58 also passes through inverter 70 to resetflip-flop 72 for preventing it from generating a "write fault" at wire37.

From the foregoing, it is apparent that nothing can be written into theprotected device unless a conscious effort is made to unblock the accessto it. The protection may be added quickly and easily, to almost anyexisting system, by plugging the device into data busses 38, 39.

Those who are skilled in the art will readily perceive how to modify theinvention. Therefore, the appended claims are to be construed to coverall equivalent structures which fall within the true scope and spirit ofthe invention.

The invention claimed is:
 1. A write protect system comprising:acomputer having at least one memory means, said memory means including ahard disc drive, a controller coupled to said hard disc drive by atleast one information transfer bus and at least one control data bus,means for selecting said hard disc drive for operation by sending asignal from said controller to said hard disc drive on a first wire insaid control data bus, means for enabling said hard disc drive to erasedata in said hard disc drive by sending a signal from said controller tosaid hard disc drive on a second wire in said control data bus, circuitmeans coupled to said control data bus for monitoring signalstransmitted on said first wire in said control data bus, guard means forselectively interrupting the signal from said enabling means on thesecond wire to prevent said hard disk drive from being enabled to erasedata thereby preventing data from being accidentially erased from saidhard disc drive, said guard means including a plurality of logicdevices, at least one of said logic devices having an output coupled tosaid hard disc drive and at least one of said logic devices having aninput coupled to said enabling means by connection to said second wire,and a manual switch coupled to said logic devices for controlling thelogic states of said logic devices to enable gating of the signal fromthe enabling means responsive to the manual switch, whereby data cannotbe erased from said hard disc drive when said manual switch is in aselected write protect state.
 2. The system of claim 1 comprisingmeansfor indicating to said controller that a fault has occurred and saidhard disc drive has not responded to said selection means or saidenabling means, said fault indicating means sending a signal from saidhard dic drive to said controller on a third wire in said control databus, and means for creating a fault indication when said enabling meansis enabled from said controller and said guard means interrupts saidenabling means, preventing said hard disc drive from being enabled. 3.The system of claim 1 whereinsaid guard means includes a first gatehaving first and second inputs and an output, the first of said inputsbeing connected to said manual switch, the second of said inputs beingcoupled to said first wire, and said output being connected to a firstinput of a second gate, said second gate also having a second input andan output, said second input of said second gate being connected to saidsecond wire to receive signals from said controller, said output of saidsecond gate being connected to said second wire to send signals to saidhard disc drive, said first gate producing a first selected output whensaid manual switch is in said write protect state and said hard discdrive is selected by said select means, said second gate producing asecond selected output when said first gate produces said first selectedoutput, regardless of whether signals are sent from said controller onsaid second wire.